1. Field of the Invention
The invention relates in general to a dual-band filter, and more particularly to a dual-band bandpass filter.
2. Description of the Related Art
The wireless local area network and mobile communication applications are growing wider and wider, and the development of the applications is advanced toward the multi-band multi-mode system. The dual-band bandpass filter is an important key component in a specific multi-band multi-mode radio frequency transceiver circuit. FIG. 1 shows the architecture of a concurrent dual-band receiver, which is proposed by Hossein Hashemi in “IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 50, NO. 1, JANUARY 2002,” and entitled “Concurrent Multiband Low-Noise Amplifiers-Theory, Design, and Applications.”
FIG. 2 is a circuit diagram showing a dual-band multilayer bandpass filter disclosed in U.S. Pat. No. 6,147,571, wherein the dual-band filter is implemented by combining two filters with different frequencies in a multilayer board structure in conjunction with a matching circuit composed of a capacitor and an inductor at input and output ends. However, the additional matching circuit required in this dual-band filter causes relative difficulty in the practical design and circuit layout stage.
FIG. 3 is a schematic illustration showing a dual-band filter applied to a multi-mode radio system, which is proposed by Sheng-Fuh Chang in “ELECTRONICS LETTER, VOL. 41, NO. 1, JANUARY 2004,” and entitled “Dual-band step-impedance bandpass filter for multimode wireless LANs.” This dual-band filter adjusts the length and impedance of a half wavelength transmission line resonator so as to obtain two pass bands. However, to flexibly control the secondary resonance mode of step impedance half wavelength resonator, the impedance ration of the resonator might meet with the limitation of circuit implementation, thereby causing difficulty in the utility stage. In addition, the dual-band filters are mostly implemented on a single layer printed circuit board. So, when the dual-band filter layout is to be performed on a multilayer board, a large area will be wasted and the specification of the general commercial components or modules cannot be satisfied.